It is known that a polyurethane elastic material is produced by an extrusion molding method using thermoplastic polyurethane pellets produced from a polymer diol, a diisocyanate and a low molecular weight diol. However, as part of bonds in the polymer decompose during extrusion molding, a polyurethane elastic material which has satisfactory thermal properties such as residual starting permanent set after elongation in a high-temperature environment, especially polyurethane yarn having excellent thermal properties cannot be obtained by this method.
There are also known reaction spinning methods for the purpose of improving the thermal properties of polyurethane elastic yarn by a melt spinning method, such as one method in which a polymer diol, a diisocyanate and a low molecular weight diol are polymerized by a one-shot process and yarn is directly spun from this polymerization system, another method in which a prepolymer having isocyanate-terminal is reacted with a low molecular weight diol and yarn is directly spun from this reaction system. The elastic materials obtained by these reaction spinning methods are superior in thermal properties to an elastic material produced from pellets by a melt spinning method due to the low thermal history of the polymer. That is, in the melt spinning method using pellets, an isocyanate remaining in pellets after synthesis reacts with water contained in the air to form an urea group or a 3-dimensional bond by the reaction of an allophanate or biuret. The urea group and the 3-dimensional bond improve the thermal properties of the polymer but decompose during extrusion molding. It should be said that an elastic material obtained by a reaction spinning method can retain excellent thermal properties because the urea group and the 3-dimensional bond remain in the polymer as they are. However, as the reaction spinning method itself is liable to spin unstable polymer which is incomplete and in course of polymer synthesis, it is inferior in spinnability. Further, since a reactant is reacted with another reactant which greatly differs from it in viscosity and volume ratio while being mixed together, a large number of abnormal reaction products are easily produced by nonuniform mixing and it is difficult to measure a trace component accurately, whereby spinning stability is impaired and it is also difficult to obtain uniform polyurethane elastic yarn, especially uniform polyurethane elastic yarn of small denier.
In order to further improve the thermal properties of a polyurethane elastic material produced by a reaction spinning method, it is generally effective to increase the molar ratio of a diisocyanate to the total of a polymer diol and a low molecular weight diol. However, when the molar ratio of the diisocyanate is increased, the molecular weight of a polyurethane polymer does not become sufficiently high at the time of spinning, resulting in reduced spinnability. Therefore, the inventors of the present invention have proposed a method for improving spinnability by adding a special additive such as a diisocyanate dimer or fine powder silica to carry out a reaction spinning method for obtaining polyurethane elastic yarn from a prepolymer having isocyanate-terminal and a low molecular weight diol (Japanese Patent Publication Nos. 63-53287 and 63-53288). However, this method has a problem with the uniform dispersibility of these additives and such problems as a complicated process and difficulty of selecting mixing conditions at the time of a reaction.
There is also known a method for obtaining an elastomer such as a pellet by reacting a mixture of a polymer diol and a low molecular weight diol in an amount 1 to 3 times the molar amount of the polymer diol with an isocyanate to give a prepolymer having hydroxyl-terminal and a prepolymer having isocyanate-terminal and then reacting the two in order to reduce a difference in viscosity between the prepolymer having hydroxyl-terminal and the prepolymer having isocyanate-terminal so as to make mixing state well (Japanese Patent Publication No. 43-639). However, the elastomer obtained by this method is an elastomer for melt spinning which is molten and spun into yarn and it is not disclosed in the above publication that spinning is carried out by reacting raw materials such as polyols continuously.